While native p2 I -ras is an essential growth-regulating component of eukaryoticcells, single point mutations are sufficient to convert the cellular form into a tumor promoting oncoprotein. In a high percentage of human tumors, one of the three endogenous ras genes is activated by a somatic point mutation. Activating mutations in principle can interfere with both regulating mechanisms: either nucleotide exchange is enhanced or GTP-hydrolysis is drastically slowed down. Over the past few years, X-ray diffraction studies on p2l have contributed greatly to the understanding of the structure and mechanism of action of this small GTP-binding protein. The structure of the GppNHp complex of p2l-ras has been refined at 1.35 A resolution. Preliminary experiments performed at the Photon Factory synchrotron, Tsukuba, Japan, indicate that diffraction data might be collected to better than I A resolution, there was a weak signal to 0.9 A and plots of R-syms. and R-cryst were smooth to a resolution of 1. 1 A. The data were collected under non-optimal conditions and the crystals were not frozen, so radiation damage was a problem and our analysis had to be done on incomplete data sets. In the meantime, preliminary flash freezing tests have been done showing that the crystals can be frozen without loosing resolution as far as can be assessed on a rotating anode source. It is helpful that the crystals are actually grown in a cryo-protectant solution. We will aim to measure diffraction the diffraction pattern of such a crystal to the limitof resolution, hopefully well below I A. Data collection on BioCARS Station 14-BM-D.